SA Node.
The sinoatrial node or SA node is a natural cardiac pacemaker which is lodged in the upper right corner of the wall of the left atrium near the opening of the superior vena cava. It is made up of highly specialized cells or neurons which spontaneously generate electric impulses, which set off a sequence of electrical activity in the heart to initiate the normal sinus rhythm and result in the sequence of the cardiac cycle. It is important that you read control of heart which describes the electric conduction system of heart.
Sinoatrial Node – Pacemaker of Heart.
The Sinoatrial node is self firing and undergoes repeated cycles of depolarization followed by repolarization of its cell membranes. It is made up of cells which are modified cardiomyocytes. They do not contract though they contain contractile elements.
Picture of SA node.
This electric impulse generated by the sinoatrial node is transmitted throughout the wall of the right atrium and through the Bachmann’s bundle to the left atrium, stimulating the myocardium of both the atria to contract. About 100 million myocardial cells in both the atria contract in less than one third of a second. This conduction of electrical impulse throughout both the atria is reflected as the “P” wave on the electrocardiogram (ECG).
Though all the cardiac cells can generate such electric impulses to cause myocardial contraction, the sinoatrial node initiates the contractions simply because it generates the impulses at a faster rate. This is because the heart cells have refractory periods after a contraction during which they cannot initiate another contraction. As a result, the SA node and AV node take priority over the pacemaker potential of the cardiac cells.
The SA node initiates the electric impulses and is responsible for the heart’s electric activity. It is therefore called the primary pacemaker. It discharges impulses which make the heart beat at normally 60 to 90 beats per minute.
Non Function of SA Node.
If the sinoatrial node does not function for some reason or its impulse is prevented from proceeding down the electric conduction system, the cells of the AV node take over, though the impulses per minute are less than those generated by the SA node. The AV node is situated between the atria and the ventricles inside the left atrium.
If the AV node also fails, the cells of the Purkinje fibers can also generate electric impulses, but they do not regulate the heart rate because their impulse generating frequency is very slow.
A sinoatrial dysfunction results in faulty electric signals resulting in irregular heart beats. It can result in tachycardia (faster heart beats) or bradycardia (slower heart beats).
Blood Supply of SA Node.
In most individuals, the SA node is supplied blood by the right coronary artery. Occlusion of this artery can cause ischemia of the SA node unless there is collateral circulation aided by the left coronary artery. Ischemia of the SA node can result in death of its cells and failure to generate impulses. In such cases, as mentioned above, the AV node takes over the pacemaker function, though at a slower rate.
Nerve Supply of SA Node.
The sinoatrial node is innervated by both the sympathetic ( Spinal nerves from T1 to T4) and the parasympathetic nervous system (Vagus nerve).
- Parasympathetic nervous system. Stimulation of the vagus nerve of this system causes the SA node to slow down its rate of impulses thereby decreasing heart rate.
- Sympathetic nervous system stimulation results in increased frequency of impulses resulting in increased heart rate. As the nerve fibers of this nervous system also innervate the atria and the ventricles, stimulation of the fibers of the sympathetic nervous system also increases the force of the contraction.
Your action step.
If you feel this article could be useful, do share it on social media and pass a comment below.
Medic On WebRelated posts: